A pest, when we are talking about gardening, is usually something we want to “take care of,” and usually right now! The problem is “right now” may not be the best time to be effective in taking care of that pest.

It’s often too late by the time “right now” occurs — which is often when a weed or an insect “suddenly” takes over. Usually that’s not what really happened — it just seems that way. Most often, it takes awhile for the pest to reach a population level that causes significant damage; by then, the pest may not even be controllable.

We should try to target pests at a time when either they are more vulnerable, or when the population is small enough that control is more effective at reducing a future population increase. The big question: when is that? How do you know when to use a control in order for it to be most effective?

That’s where Growth Degree-Day models come into play. Knowing the accumulation of thermal heat units during the season can help you determine a pest’s life or a plant’s current growth stage. With this knowledge, you can track the accumulation of GDD (using observed temperature) to predict when to plant seeds, apply fertilizer or pesticides, and even estimate when to harvest crops. GDD data and models can even be useful for determining which plants are most suitable for a given climate, timing planting to avoid frost damage and predicting the current life stages of insects.

GDD is a calculation of the amount of total heat accumulation above a set base temperature over time. This is known as a growth degree day unit, or GDD. The GDD model varies by the type of pests or plants; some need more growth degree units, while some need less. The base temperature is that temperature below which plant or pest growth rate is zero.

The GDD formula is pretty simple: high daily temperature + low daily temperature, divided by 2, equals the average daily temperature. Next, subtract the base temperature from average daily temperature to get the total growth degree day units for that day. If the number is a negative, you record a zero for that day, and if it’s a positive you add it to the accumulative total GDD for the year. The base number will vary by pest or plant species.

Using this type of model helps account for the weather variations from year to year, rather than using a historical calendar date for planning purposes to determine when to apply a treatment method or expect a crop. You may want to explore this more than I can cover in this article, so here are some sites that you might look for some of those answers:

Both sites show GDD temperatures and a variety of models for different plants and pests.

A plant’s bloom time may be the right time to help me make decisions about other pests without having to do the math. For instance, forsythias tend to bloom at about the same time as pre-emergence herbicide applications for crabgrass need to be applied. While this is not a calculation, it is a reminder the GDD unit for crabgrass germination, which is around 200 GDDs, is getting close. Remember most pests don’t suddenly appear; they just reach a damage threshold that brings them to your attention. They are much easier to control before the numbers reach that level.

A WSU Master Gardeners of Chelan County column appears weekly in The Wenatchee World. Lloyd Thompson is one of four columnists featured. Learn more, visit wwrld.us/cdmg or call 667-6540.